1. Field of the Invention.
This invention is concerned with the preparation of single crystals of refractory metal halides and rare earth halides by a reactive atmosphere processing method and their use as laser windows.
2. Description of the Prior Art.
Laser window technology has been concentrated primarily in the development and evaluation of three classes of compounds: halide salts, semiconductors and non-oxide glasses. Compounds of these classes, while interesting for laser window applications, are limited by their optical absorption, mechanical or thermal characteristics. Materials which are transparent at a desired wavelength are often eliminated from consideration because they fail in one or the other of the necessary properties. The alkali metal halides generally exhibited the lowest optical absorption coefficients; however, their thermal and mechanical properties are such that the system reliability is degraded. While the semiconductors possess superior thermal and mechanical properties, they often exhibit high optical absorption coefficients when utilized for laser operations in the 10.6 .mu. region. Within the alkali metal halide class of compounds the most outstanding laser window material has been found to be potassium chloride (KCl). Gallium arsenide has been found to be the best laser window material within the semiconductor class ("Windows for High Power Lasers" by F. Horrigan et al, Microwaves, Jan. 1969).
It has been long recognized that one of the limiting factors affecting the usefulness of alkali metal halide crystals as laser window material is the purity of the crystal itself. Evidence has been obtained which shows that further improvements in optical absorption and mechanical integrity can be expected from technological advances in surface preparation. The improvements were noteworthy in that they were realized in the purified material, and were superior to the improvements obtained earlier by bivalent metal ion doping ["Strain Aging in CdCl.sub.2 -Doped Rock Salt" by L. M. Brown and P. L. Pratt, Phil, Mag. 8, 717 (1963)]. However, it has been shown that the doping technique of material purification, while upgrading the mechanical properties of the materals, exacts a tradeoff in optical absorption (.beta.) ["Impurity-Induced Infrared Absorption in Alkali Halide CO.sub.2 Laser Windows" by F. W. Patten et al, Mat. Res. Bull. 6, 1321 (1971)]. Bivalent cation impurities stabilize the hydroxide ion (OH.sup.-) and cation vacancies in alkali metal halides ["Der Einflusn von OH.sup.- -loen and Absorptions Spektrum und ionenleitfahigheit von KCl-Einkristallen" by B. Fritz et al, 2. Phys. 174, 240 (1963)].
The inability to effectively remove the hydroxide ion (OH.sup.-) by the doping technique is probably due to its pseudo halide character in that the reactivity of the OH.sup.- is very close to that of fluoride. (See FIG. below.) ##SPC1##
From the standpoint of optical transmission, the O--H vibration is active in the 2-6 .mu.m region and the O--M at the 9-12 .mu.m region: The substitutional OH.sup.- dipole couples with the dipole consisting of a bivalent metal ion impurity and a metal ion vacancy [B. Fritz, Supra, "Influence of OH.sup.- Ions on Infrared Absorption and Ionic Conductivity in Lithium Fluoride Crystals" by T. G. Stoebe, J. Phys. Chem. Solids 28, 1375 (1967)]. Through such couplings and the tendency to diffuse to the surface, the OH.sup.- can effect a degradation of the mechanical behavior and surface stability of alkali halides. Achievement of OH.sup.- control in the growth of metal halide crystals has therefore constituted a major limitation to the use of these materials for laser window applications.
The concept of growing an alkaline earth fluoride crystal, a rare earth fluoride crystal or a crystal of alkaline earth fluoride-rare earth fluoride under an atmosphere containing hydrogen fluoride in order to obtain laser quality crystals which are purified of oxides and oxyfluorides is disclosed in U.S. Pat. No. 3,649,552. Our invention constitutes a significant improvement over this patent in that our process is effective in the elimination of OH.sup.- ion impurities from crystals prepared by conventional methods to yield ultra high purity crystals that have excellent properties for utilization as 2- 6 micron range laser window material. Our invention differs from that of U.S. Pat. No. 3,649,552 in several ways. First, we start with a conventionally grown alkaline earth fluoride crystal having known characteristics and significantly improve these characteristics by recrystallization and regrowth. Secondly, we utilize the RAP method whose chief objective is to reduce as low as possible, the pressure ratio of H.sub.2 O/HF to obtain single crystals of pure alkaline earth fluorides rather than doped mixtures as taught by the patent. The control of H.sub.2 O/HF in the gas phase, as taught by our invention, provides the key to the control of OH.sup.- in the crystal which, as we have previously pointed out, can cause an increase in the optical absorption in the infrared. When the concentration of OH.sup.- in the crystal reaches a saturation value, precipitation of a hydroxide or an oxide phase occurs. This is seen as a turbidity which is very much undesired for the use of the crystal as a laser host. (The suspended particles are scattering centers.) The concentration of OH.sup.- in the crystal can be quite below saturation, rendering the clear crystal satisfactory as a laser host, but still too high for its application as a laser window in the infrared region where the OH.sup.-renders the material optically absorbent at 2-6 .mu.m and 9-12 .mu.m.